Insulated wires such as those used in communications cable often include flame retardant insulating materials. These flame retardant insulating materials allow these cables to be located in plenum air spaces of buildings or in other locations where flame retardancy and low smoke generation are important properties for the cable. In communications cables, these insulated wires are often provided as twisted pairs consisting of two insulated conductors twisted about each other to form a two conductor group.
The flame retardant insulating materials conventionally used include fluorinated polymers such as fluorinated ethylene-propylene (FEP), ethylenetrifluoroethylene (ETFE), and ethylenechlorotrifluoroethylene (ECTFE). Although these fluorinated polymers impart the necessary flame retardant properties to the plenum cable, these polymers are generally quite expensive. Therefore, it is desirable to minimize the amount of fluorinated polymer material used to insulate conductors for communications cables.
One method for minimizing the amount of insulating material used to insulate conductors is to foam the polymer insulating material. Foaming the insulating material also has the benefit of improving the electrical transmission characteristics of the resulting cable. Typically, the insulating materials are foamed using a gas blowing agent such as nitrogen or carbon dioxide. The conventional method of using gas blowing agents for insulation is to feed the polymer insulating material to an extruder and inject the gas blowing agent into the polymer melt. The polymer insulating material and blowing agent are then blended and a layer of the insulating material is applied around the conductor. Preferably, the insulating material is applied as a thin layer to further reduce the amount of insulating material used in the cable.
Although this is a common method for applying insulating material to conductors, there are problems associated with foaming polymer insulating material with gas blowing agents. In particular, it is difficult to control the amount of gas blowing agent fed to the extruder. Therefore, if a thin insulating layer of the foamed polymer is to be applied to the conductor, small variations in the process conditions often occur which result in disproportionately large changes in the characteristics of the foamed polymer. For this reason, it is difficult to maintain close manufacturing tolerances for density, thickness, dielectric constant, etc. This is particularly a problem at the high temperatures used to melt fluorinated polymers. As a result, it is difficult to produce a layer of foamed fluorinated polymer insulating material having uniform or consistent properties along the length of a wire. Therefore, the electrical properties of the insulated conductor and the cable suffer.
An additional problem that is encountered in using gas blowing agents and in particular, nitrogen, for foaming fluorinated polymers is that the cell size of the resulting polymer insulation is too large for thin insulating layers such as 30 mils or less. As a result, there are breaks in the insulation thereby affecting the insulative properties of the fluorinated polymer layer.
Another problem associated with using gas blowing agents is that the small port used to inject the gas blowing agent into the extruder often becomes blocked by the polymer material or by airborne dust and dirt. As a result, the extruder must be taken off-line and the port cleaned thereby preventing the operation of the insulating process. Therefore, there is a need to find an alternative to using gas blowing agents to produce foamed fluorinated polymer insulation for insulated conductors.